1. Field of the Invention
The present invention relates to an equalization apparatus such as maximum likelihood sequence estimator (MLSE) and the like for use in mobile communication terminals, and an equalization method.
2. Description of the Related Art
Conventionally, in mobile communication, multipath is formed by reflection, diffraction and scattering by buildings, producing multiplex signal interference and multipath fading where the amplitude and phase of received signals fluctuate when a mobile communication terminal moves. In addition, when differences in the time of arrival due to propagation delay are not negligible, intersymbol interference occurs where a preceding symbol interferes with a succeeding symbol. Hence, an equalization apparatus is needed to minimize influences of intersymbol interference included in the received signal and accurately estimate the transmission symbol sequence.
In mobile communication of recent years, the equalization apparatus is indispensable with increase in transmission speed. However, power consumption of the equalization apparatus upon signal reception is substantial, which greatly influences talk time and standby time of mobile communication terminals powered by a dry cell or battery, and results in a technical problem of difficulty of long time use. So far, a low power consumption method improving this point is proposed in document 1 (Japanese Patent Application Laid-Open No. HEI8-23282).
FIG. 6 is a block diagram showing a schematic configuration of an equalization apparatus of a mobile communication terminal disclosed in document 1. The equalization apparatus shown in this figure has: received signal storage section 200 that stores received signals; channel response estimator 201 that estimates a channel response; control circuit 202 that controls the number of states employed in maximum likelihood sequence estimation; and a Viterbi equalizer 203 whereby the number of states is variable, and sets a power level threshold for channel response values, makes channel responses greater than the threshold valid, and determines the number of states based on the result, thereby reducing the amount of equalization processing.
However, with the conventional mobile communication terminal, an equalization apparatus sets a power level threshold for channel response values, searches for a response having the largest delay and larger power level than the threshold, makes responses up to that response valid and determines the number of states in an equalizer. Thus, although low power consumption is achieved by reducing the operation amount of equalization processing, there is a problem that the power level threshold cannot be adaptively updated in accordance with radio channel conditions.
In addition, since the power level threshold does not follow radio channel conditions, optimization of the number of states and consequent equalization processing is not carried out. There is therefore a problem that low power consumption is not achieved adequately.
Furthermore, since the power level threshold does not follow the radio communication channel, an error in a decision value after equalization processing may occur. There is therefore a problem that reception performance may be degraded.